This invention relates to thermocouples and, more particularly, to thermocouple structures especially suited for use in the pilot burner of a gas heater.
Conventionally, the thermocouple used in a target-type pilot burner, of which the pilot burner disclosed in U.S. Pat. No. 3,291,185 issued to Harold A. McIntosh et al., is typical, comprises a length of wire and a length of tube made from dissimilar metals. The length of wire, which has an appreciably smaller diameter than the inside diameter of the tube, lies inside the tube in axial alignment therewith. Preferably, the extremity of the tube against which the flame of the pilot burner impinges has a smaller diameter and wall thickness than the base of the tube so as to promote large heat transfer radially inward through the extremity and small heat transfer axially from the extremity to the base of the tube. A tip is formed on the end of the extremity of the tube to seal its interior from the atmosphere and to form a hot thermocouple junction between the extremity of the tube and the adjacent end of the wire. A pair of leads, permanently joined respectively to the base of the tube and the adjacent end of the wire, connect the thermocouple to an electrical measuring instrument, or electrical coil, operating a control device.
The tube is made from a nonoxidizable metal such as stainless steel, but the wire is generally made from an oxidizable metal. If the seal at the extremity of the tube is imperfectly formed by the tip, oxidation of the wire takes place and the hot thermocouple junction deteriorates. The diameter of the wire is appreciably smaller than the inside diameter of the tube to avoid the possibility of a short circuit through contact between the wire and the tube at a point other than the hot junction. As a result, it is difficult to center the end of the wire sufficiently in the open area at the end of the extremity of the tube while the tip is being welded. Eccentricity between the end of the tube and the wire tends to cause an uneven distribution of the metals in the metallic weld bead forming the tip. As a result, stringers of the wire material may be produced in the tip that are eventually oxidized, leaving the tip porous. If more heat is used to form the tip in an attempt to eliminate imperfect seals, the danger exists that too much admixture of the dissimilar metals occurs at the tip to form an efficient hot junction. The problem of imperfect seals at the tip becomes especially acute if the tip forming operation is automated by use of heliarc equipment.